When the electric motors within machine tool cutting operations are reversed to change the direction of the cutting tools, large inrush currents are temporarily generated. To prevent circuit interruption each time the operations are reversed, fuses are generally required in series with thermal overload relays, which protect the motor and associated equipment upon the occurrence of a true overload condition. The fuses, however, must be replaced on a periodic basis due to thermal aging when subjected to repeated high currents. With the introduction of high efficiency motors that operated at lower operating currents and higher inrush currents, higher fuse ratings are selected to sustain the higher inrush currents and resist thermal aging. U.S. patent application Ser. No. 08/262,394, filed Jun. 15, 1994 entitled "Induction Motor Protective Circuit Breaker Unit" describes the selection of the fuse rating and circuit breaker parameters to reduce the thermal damage to the fuse and to allow coordination with the thermal overload relays commonly employed within electrical motor protective circuits. However, such higher-rated fuses find limited application outside the tool-making industry.
U.S. Pat. No. 4,884,164 describes an industrial-rated circuit breaker having an electronic trip unit that is adjustable over a wide range of long and short time over-current conditions.
U.S. Pat. No. 4,967,304 describes a digital circuit interrupter with electric motor trip parameters that includes control algorithms tailored for motor protection applications.
U.S. patent application Ser. No. 08/189,613 filed Jan. 31, 1994 entitled "Current Limiter for Molded Case Circuit Breakers" describes a current suppressing unit which rapidly suppresses the short circuit current to a substantially lower value without fusing.
It would be economically advantageous to have a combined electronic trip circuit breaker and current suppressing unit which is not susceptible to thermal aging, does not fuse, and does not require periodic replacement when used in tool making operations.
One purpose of the invention is to provide a combined circuit breaker and current suppressing unit that does not interrupt the protected circuit upon motor reversal and can be adjusted to provide coordination with the thermal overload relay.